When a guy gets excited there will be pre-come which contains sperm cells. Only one fast one is needed to become pregnant. A guy has no control over this pre-come.
Many guys have no voluntary control over their orgasm. Some guys are able to determine when they come, but with many orgasm sneaks up on them. So they have no way to be careful.
Even if a guy has control and feels his climax coming, he might be tempted to forget his good intentions when the moment arrives.